This invention relates to hot-fill plastic or polyester containers, and more particularly to such a container having an improved sidewall construction.
In the past, most plastic or polyester containers were used to contain liquids that are initially dispensed at room temperature or chilled. However, in recent years, there has been a significant increase in the demand for polyester containers for packaging "hot fill" beverages. "Hot-fill" applications impose additional mechanical stresses on the container structure which cause the container to be less resistant to deformation when the container is being handled or if it is dropped. The thin sidewalls of conventional polyester containers deform or collapse at hot fill temperatures. Moreover, the rigidity of the container decreases immediately after the "hot-fill" liquid is introduced into the container, making the container more susceptible to failure due to mechanical stresses. As the hot-filled liquid cools, it shrinks in volume which has the effect of producing a negative pressure or "hot-fill" vacuum in the container. The container must be able to sustain such internal pressure changes while maintaining its configuration.
Various methods have been devised to counter thermal instabilities. One method broadly involves heat treating the polyester to induce molecular changes which will result in a container exhibiting thermal stability. Other methods involve forming the polyester structure into a structural configuration which can maintain stability during hot fill. Thus, the hot-fill containers being produced have a generally cylindrical main body which is provided with a plurality of elongated vertically oriented panels. These panels, which are commonly referred to as pressure or vacuum panels, are designed to collapse inwardly after the container has been filled with a hot liquid so as to accommodate the inevitable volume shrinkage of the liquid in the container as the liquid cools. However, the inward flexing of the panels caused by the hot fill vacuum creates high stress points at the top and bottom edges of the pressure panels, and especially at the upper and lower corners of the panels. These stress points weaken the portions of the sidewall near the edges of the panels, allowing the sidewall to collapse inwardly during handling of the container or when containers are stacked together.
This problem could be alleviated by increasing the thickness of the container wall. However, increasing the wall thickness results in an increase in material cost for the container and in the weight of the finished container, which results are not acceptable to the container industry. The effects of hot-fill stresses can be minimized by providing pressure panels which extend substantially the entire vertical length of the sidewall and with longitudinally extending ribs extending along the edges of the panels. Examples of containers of this type are shown in U.S. Pat. Nos. 4,805,788 and 4,863,046. The hot-fill container disclosed in U.S. Pat. No. 4,863,046, for example, has a cylindrical main body portion which includes a plurality of vertically oriented pressure panels separated by vertically elongated land areas. The pressure panels extend from just below the label upper bumper to just above the lower label bumper. The vertically elongated land areas between the pressure panels are reinforced by vertical ribs. Each of the pressure panels includes a plurality of transverse, horizontally extending radially recessed rib segments within the panel which ensure that the panel moves uniformly. Because the pressure panels extend from just below the upper label bumper to just above the lower label bumper, the area for securing the label to the container body is minimized. This imposes significant constraints on the manufacturing tolerances in applying the label to the container. Label placement is critical because the areas above and below the panels for placement of the upper and lower edges of the label are relatively small. Moreover, because the size of the panels is large relative to the label bearing portion of the sidewall, there is a minimal flat cylindrical area for securing the label to the container.
Another container construction for hot-fill applications, which is disclosed in U.S. Pat. No. 5,067,622, has its sidewall rigidized by a plurality of concentric rings which prevent inward flexing of the sidewall. The container includes a plurality of small vacuum panels in the neck portion of the container. The vacuum panels in the neck portion of the container and a vacuum panel in the base of the container permit the container to deflect under hot fill and subsequent vacuum conditions. However, the body portion does not undergo either radial or longitudinal contraction, and the vacuum panels work independently of the sidewall reinforcement. Moreover, this construction results in a minimal flat cylindrical sidewall area for receiving the upper and lower edges of the label so that label placement is critical.
Another hot-fill container, which is disclosed in U.S. Pat. No. 4,749,092, includes a sidewall portion which contains a plurality of pressure panels and a smooth surfaced cylindrical portion, which is located above the pressure panels, which is adapted for affixation of a label. In one embodiment, the label receiving portion includes annular grooves which reinforce only the label receiving portion of the container, and the label does not cover the pressure panels.